Unit:-3
Water Demand
Introduction
The demand of water is the total amount of water used by the consumer with in the most important aspect before designing a water supply scheme. The units givens the idea on how to determine total water demand of the city or town or village. Various type of water demand of city or village are described in this unit with reasonable data. Population forcasting methods such as arthimatic increase method, geomaterical increase method, and incramental increase method have also been described with suitable example.
Water demand
When designing the water supply scheme for a city or village it is necessary to determine the total quantity of water reequried for various purpose by city or village. The Duty od water engineer is to place determine the water sources from where the demand can met.
The determination of water demand involves the following items:-
1.Rate of demand
2.Population determination geomaterical
Type of water demand
Following are the various type of water demand of a city or village:-
1.Domestic water demand
2.Commercial and Indrustial water demand
3.Fire demand
4.Demand of public use
5.Institutinal water demand
1.Domestic water demand
It includes the quality of water requried in the house for drinking, bathing, cooking, washing etc. The quantity of water requried for domestical ue mainly deponds on the habits, social status, climatic condition and costumes of people. The ware requried for domestic purpose is given:-
1.If the population of community is less than 70000 then liter per capita per day (LPCPD) is 45 liter supply through public tap.
2.If the population of community is less than 70000 then liter per capita per day (LPCPD) is 70-100 liter supply through private tap.
3.If the population of community 70000 -100000 then liter per capita per day (LPCPD) is 100-150 liter supply through private tap.
4.If the population of community more than 100000 then liter per capita per day (LPCPD) is 150-200 liter supply through private tap.
2.Commercial and Indrustial water demand
The presence of indrestries in the city has a great effect upon total comsuption of water. There is no direct relation of this comsumption with the population and hence, actual requriment for all indrestries should be carefully estimated. The water requriment in the indrestries may depond upon type and size of indrestries. The water requried by the indrestries like paper mils, cloth mills, suger refineris etc comes under indrestial use of water. The quantity of water demand for indrestials use is around 20 to 25% of total water demand of the city.
Commercial building and Commercial center include offices building, hotels, schools cinema hall, bus station, health centers, airpot etc. The requriment of commercial and public place may be upto 45% per capita per day. The water requriment for commercial and indrestial demand are given below:-
1.In Health post Liter per capita per day (LPCPD) is 500 liter/bed or 3000 liter/day if not beded health post.
2.In Public instution Liter per capita per day (LPCPD) is 100-500liter/day.
3.In Hotel/Resturant Liter per capita per day (LPCPD) is 100-500liter/day.
3.Demand for public use
Quantity of water requried for public utility purpose such as for washing and sparking of road, cleaning of sewers, watering the parks etc comes nder public demand. Usually 5% of the water demand of city should be consier for public use.
4.Fire demand
The water requried for fire fighting is generally known as fire demand. It is relate as a function of population and maybe computed from the following:-
Empical furmula:
1.Kuichling's formula
Q=3182√P
2.Buston's formula
Q=5663√P
3.Freeman's formula
Q=1136[P/5 + 10]
where,
P=population in thousand
Q=Quantity of water in liter per minutes
5.Compensate losses demand
All that water, that goes into the distrubition pipe doesn't reach the consumers. Some portion of water is wasted in the pip lines due to effictive pipe joints, cracks and broken pipe fautly valves and fitting. Sometimes comsumers keep open their tape or public taps when they are not using water. While estamating total requriment of water of a town or city compensation for water loss and wastage should be done. Generall 15% of the total quantity of water needed to compensate for losses, thefs and wastage of water.
6.Per capita demand
Per capita demand means the average water for each person in a particular comminity uses in a daily basis. It is expressed as liters per capita per day. If Q is tthe total quantity consumed by a city per year in liter and population is p there per capita demand will be:-
Per capita demand :-Q/px365 liter/day
7.Design period
The extensions in the water work, networks of pipe line etc is not an easy work. Thr design and construction should be such that it could meet demand easily. The number of years for which the design of water works ha been done is known as design period. In another word design period refers to the duration for which a scheme will met water demand of water users. Mostly in Nepal, water worls are design for design period of 15-30 years. The following factor should be kept in the mind while fixing the design period:-
1.Fund available for the complesion of project. The more funds we have less the design period.
2.Life of pipe and other structure materials used in the water supply scheme.
3.Design period is in no case should have more life than the component and materials used in the scheme
4.As far as possible designe period should be nearly equal to the materials used in water supply works.
5.If the rate of intrest is less it will be good to keep design period more, but if the intrest rate is very high the design period should be small.
6.Rate of population growth
Factor's affecting water demand
It has been noted thaht the average er capita demand for various city considerbaly varies. The following are the main factors which affects the per capita demand of the city:-
1.Cilimatic conditions
2.Size of community
3.Class of consumer
4.Quality of water
5.Cost of water
6.System of sanitation
7.System of water supply
8.use of metering System
Climatic condition
A large quanity of water is requried in the hotter and dry place because the comsumpution of water in these area are hig due to use of cooler, Ac more washing of clothes, bathing etc. In cold places the quantity od water requried is less.
Size of community
The per capita demand of city increses with a Size of city beacuse much water will be requried in the street washing . sewers, maintaince of parts and garden.
Class of consumer
High class family comsume more water due to their beter living standered. Higher class consumers requried more water than of lower class consumers. They need extra water in various cleaning purpose.
Quality of water
Quality of water is the most important point to be be consider while selecting source. The quality of water in the source should be good. So, that it can be treated easily at low cost. It should not contain diseases, germs and pathogenic bacteria which may indanger the public health.
Cost of water
Cost of water affects the water demand higher the cost of water lower the cost f the lower the demand. If all other condition remain same.
System of sanitation
The per capita demand of city having water carraiage system will be more than the city these system is not being used.
System of water supply
There are two types of water supply system continuous, and intermitent. In continous system water supply is for 24 hours. While in intermitent in system water is supplied for limited hours. The amount of water used in contionus system is more than the intermitent system due to careless of consumers.
Metering system
Water supply scheme having non-metering system requrie more water than the metering system because in metering system consumer have to pay certain cost/charge for water.
Population Forecasting
Methods of population forecasting :-
1.Arthmetrical increase method
2.Geometrical increase method
3,Decreasing rate method
4.Simple graphical method
5.Comparative graphical method
6.Master plan method
7.The ;ogistive curve method
1.Arthmetrical increase method
This is the simplest method of population forecasting. In this method the increase in population from decade to decade remain constant. From the population data of previous three to four decade, the percantage determined. In this method population is forecasted by using following formula:-
Pn=Po(1+r/100)^2
where,
pn = Future population at ends of decade
po = Present Population
r = Average increasement in Population
n = numbers of years
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